Maps of the dominant period derived from the global power spectrum within the ROI during both observing intervals (upper and lower rows, respectively) for Ca II ± 0.11 Å, Ca II 8542 Å line centre, Hα ± 0.26 Å, and Hα line centre intensity variations, and for the Hα Doppler velocity and Hα FWHM variations. The overplotted yellow, red, green, and cyan circles indicate the location of the analysed conspicuous vortex flow and of the smaller swirls indicated in Fig. 1.